Tuesday, July 06, 2010

Charles Barton Takes the Offensive in Nuclear Reactor Safety

Charles Barton has been looking at nuclear reactor safety recently. He has decided that it may be time for nuclear safety promoters to go on the offensive.

A third philosophy of nuclear safety might be called precautionary or offensive safety. This approach is only possible in fluid core reactors. In particular it was investigated at Oak Ridge National Laboratory for the Molten Salt Breeder Reactor. [Offensive] safety involves the removal of highly radioactive fission products from the nuclear core. These include the gasses xenon(Xe), and krypton (Kr),. and the volatile fission products, iodine(I), tellurium(Te), cesium(Cs), and rubidium(Rb). In particular it is considered highly desirable from the standpoint of reactor control to continuously bubble xenon out of the liquid salt core fluids, with krypton following the xenon out. They can then be captured and stored. Removing the volatile fission products enhances nuclear safety, nut solves some materials problems with MSR. Tellurium is a particular problem because it contributes to some materials problems in the MSR core. The removal of the noble metals,

The removal of the noble gases, volatile fission products, and Nobel metals can be justified by safety benefits as a step designed to increase nuclear safety while lowering overall nuclear costs. In addition the continuous removal of fission products will offer significant benefits to reactor designers.

Thus Molten Salt Reactors have unique [offensive] safety features, that involve the removal of radioactive isotopes. Combining the [offensive] safety removal of radioactive gasses, and volatile fission products with an underground location location would mean in practice that further defenses against radioisotope release in the event of a nuclear accident would be unnecessary because gravity would serve as a sufficient barrier to the movement of against the movement of non-volatile radioisotopes away from the reactor hot cell. In addition, MSRs including the Liquid Fluoride Thorium reactor (LFTR) can be designed to operate with a negative coefficient of thermal reactivity, which protects the reactor from a loss of control over criticality as internal temperature rises.

MSRs can be designed to completely shut down before rising temperatures become a serious problem. In addition freeze core drain plugs offer a fall back passive safety feature that prevents reactor overheating. As core salt temperature rises past a certain point a plug of frozen salt is melted by simple heat transfer from the fluid core salt to the frozen plug salt. Once the plug melts the core salt drains into a tank or series of tanks shaped to prevent criticality. A passive air cooling system can insure that heat from the radioactive decay of the remaining fission products in the core salts will not become a problem.

The ARC-100, a proposed 100 MWe Integral Fast Reactor would share with other IFRs a negative temperature coefficient of reactivity. A large tank of liquid sodium located in close proximity to the reactor core would then serve as a thermal reservoir to prevent overheating due to the radioactive decay of the fission products embedded with in the fuel. A passive air cooling system will prevent the core and sodium tank from overheating. The ARC-100 breeding ratio is unlikely high enough to pose a void wort problem, and even if it did, Argonne National Laboratory research indicates that the IFR negative temperature coefficient of reaction feature of IFRs would shut down the fission process in the ARC-100 core before the core is damaged.

Thus both the LFTR and other factory produced MSTs as well as the ARC-100 would offer outstanding levels of safety. In particular MSR safety could offer a major route to lowering reactor price, because massive safety structures would be unnecessary. _CharlesBarton

There is much more at the link above.

The idea is to design the safety into the reactor, so that multiple complex extrinsic safety systems will not be needed. With present reactors, large numbers of trained personnel are necessary to constantly watch over safety and operational indicators -- constantly on the ready to intervene in case of problems.

But reactors which are built to keep themselves within safe limits of operation do not require expensive armies of overseers, eyes glued to screens and dials.